Boris Johnson has warned there is “no easy military solution” to prevent North Korea escalating its nuclear aggression but said all options were still on the table for retaliation after the regime’s nuclear weapons test.

The UK foreign secretary said it was not clear how a military response from the west would be possible, given the proximity of the South Korean capital, Seoul, to the North Korean border.

Any military challenge to Kim Jong-un’s regime could come at huge cost to civilian lives in the South, Johnson said.

“It’s certainly our view that none of the military options are good,” he told reporters after North Korea announced it had tested a powerful hydrogen bomb that could be loaded on to an intercontinental ballistic missile

“It is of course right to say that all options are on the table, but we really don’t see an easy military solution,” he said. Were the west to hit back with force against North Korea, “they could basically vapourise” large parts of the population even with conventional weapons, Johnson said.

“So that’s not really very easy to threaten and to deliver,” he said. “Much more productive, we think, is to continue with the international diplomatic effort.”

The foreign secretary’s comments put him at odds with the US president, Donald Trump, who ramped up the rhetoric on Sunday by accusing South Korea in a tweet of favouring a policy of “appeasement”, telling Seoul that method would not work because the North Koreans “only understand one thing!”

Johnson called for “common sense” to prevail in the crisis and urged Beijing to put further pressure on Kim’s regime. “There is no question that this is another provocation. It is reckless. They seem to be moving closer towards a hydrogen bomb, which, if fitted to a successful missile, would unquestionably present a new order of threat.

“We have to consider how to respond and it’s our view in the UK, overwhelmingly, that peaceful diplomatic means are the best.”

The foreign secretary said the UK could not accept China’s suggestion that there was an equivalence between the South Korea-American military exercises and nuclear testing conducted by North Korea.

“We don’t accept that: what the South Koreans do is entirely legitimate, it’s peaceful, it’s been going on for years, it doesn’t represent any illegal provocation of that kind,” he said.

“Our message to the Chinese is, and we are working ever more closely with them, we think there is more scope for you, the Chinese, to put economic pressure on the North Koreans. It has worked, we have seen signs in the last six months of Chinese pressure actually changing the approach of North Koreans – let’s see if we can do it again.”

Theresa May has condemned the latest nuclear test by North Korea as “reckless”, saying it is more pressing than ever to look at increasing the pace of implementing sanctions on the regime.The prime minister said the test, North Korea’s sixth since 2006, “poses an unacceptable further threat to the international community”.May reiterated the call she made with the Japanese prime minister, Shinzo Abe, during her trip to Japan last week for tougher action against Kim Jong-Un.

“I discussed the serious and grave threat these dangerous and illegal actions present with President Abe in Japan this week and reiterate the call we jointly made for tougher action, including increasing the pace of implementation of existing sanctions and looking urgently in the UN security council at new measures,” she said.

“This is now even more pressing. The international community has universally condemned this test and must come together to continue to increase the pressure on North Korea’s leaders to stop their destabilising actions.”

China’s foreign ministry said the North Korean test had come “in spite of widespread opposition from the international community ... the Chinese government resolutely opposes and strongly condemns it.”

The explosion overnight caused a 6.3-magnitude earthquake felt in Yanji, China, which is about six miles from North Korea’s Punggye-ri nuclear test site. South Korea’s meteorological administration estimated the blast yield to be five to six times more powerful than North Korea’s fifth test in September last year.